Physics and Engineering of Radiation Detection

1.3. Radioactivity and Radioactive Decay 25

T

P

1/2<T1/2

D

Activity

AD

AP

Time

Figure 1.3.7: Activities of par-

ent and daughter nuclides as a

function of time for a material

that never reaches the state of

equilibrium. The parent in such

a material is shorter lived than

the daughter.

effectiveactivity and other related quantities. The expressions for the effective half
and mean lives can be obtained by substitutingT 1 / 2 ,i=0. 693 /λd,iandτi=1/λd,i
for theith decay mode in the above equation. This gives

1

T 1 / 2 .e

=

∑n

i=1

1

T 1 / 2 ,i

(1.3.35)

and

1

τe

=

∑n

i=1

1

τi

, (1.3.36)

whereT 1 / 2 ,eandτerepresent the effective half and mean lives respectively.

1.3.G UnitsofRadioactivity

Since the most natural way to measure activity of a material is to see how many
disintegrations per unit time it is going through, therefore the units of activity are
defined in terms of disintegrations per second. For example 1 Becquerel corresponds
to 1 disintegration per second and 1 Curie is equivalent to 3. 7 × 1010 disintegrations
per second. Curie is a much bigger unit than Becquerel and is therefore more
commonly used. However for most practical sources used in laboratories, Curie is
too big. Therefore its subunits of milli-Curie and micro-Curie are more commonly
found in literature. The subunits of Curie and interconversion factors of Curie and
Becquerel are given below.